1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having large nozzles and small nozzles which eject droplets having different volumes.
2. Description of the Related Art
In a thermal-jet type of inkjet recording apparatus, which ejects droplets by using thermal energy generated by heating elements, a recording head (hereinafter, simply called a “head”) having large nozzles and small nozzles which eject droplets of different volumes is used widely in order to achieve high-definition image recording. By varying the size (surface area) of the dots formed by the respective droplets ejected from the large and small nozzles, it is possible to represent a wide variety of densities.
When using a head of this kind, the flight speed of the droplets ejected from the small nozzles is faster than the flight speed of the droplets ejected from the large nozzles, and hence there is a difference between the large and small nozzles in the time from the ejection of the respective droplets until they land on the recording medium. In particular, in a method which records by repeatedly moving the head in the breadthways direction of the recording medium (a serial scanning method), displacement arises in the deposition positions of the droplets, due to this difference in the flight speed, and hence there is a problem in that image deterioration occurs. In order to resolve problems of this kind, for example, in Japanese Patent Application Publication No. 7-137240, deposition position displacement caused by difference in the flight speed of the respective droplets ejected from the large and small nozzles is corrected by controlling the ejection timings of the large and small nozzles.
However, it has been found that there are limitations on the improvement of image quality that can be achieved by simply controlling the ejection timings of the large and small nozzles. This is because not only do the respective droplets ejected from the large and small nozzles have different flight speeds, but they also differ in terms of the length of the droplet (the length of the column of the liquid), and the number and size of satellite droplets generated concomitantly with the main droplet, and the like. Therefore, similar shapes cannot be achieved for the large and small dots formed by the droplets comprising a main droplet and satellite droplets in this way, by means of simply controlling the ejection timing so as to cancel out difference in the flight speed of the droplets, as described in Japanese Patent Application Publication No. 7-137240.
In particular, in a region where the volume of the droplets ejected from the nozzles is equal to or less than 2 to 3 picoliters (pl), the ratio of the size of the satellite droplets with respect to the size of the main droplet becomes larger and the effect of same on the recorded image becomes greater. If the satellite droplets have a great effect in this way, then complicated calculation is required in the image processing, such as halftoning, and this creates major practical problems.